Download Transition Metal-Modified Zirconium Phosphate Electrocatalysts for

catalysts
Article
Transition Metal-Modified Zirconium Phosphate
Electrocatalysts for the Oxygen Evolution Reaction
Joel Sanchez 1 , Mario V. Ramos-Garcés 2 , Ieva Narkeviciute 1 , Jorge L. Colón 2, *
and Thomas F. Jaramillo 2, *
1
2
*
Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA;
[email protected] (J.S.); [email protected] (I.N.)
Department of Chemistry, University of Puerto Rico-Río Piedras Campus, P.O. Box 70377,
San Juan, PR 00936-8377, USA; [email protected]
Correspondence: [email protected] (J.L.C.); [email protected] (T.F.J.);
Tel.: +1-787-402-2015 (J.L.C.); +1-650-353-1984 (T.F.J)
Academic Editor: Monica Pica
Received: 1 March 2017; Accepted: 20 April 2017; Published: 1 May 2017
Abstract: Zirconium phosphate (ZrP), an inorganic layered nanomaterial, is currently being
investigated as a catalyst support for transition metal-based electrocatalysts for the oxygen evolution
reaction (OER). Two metal-modified ZrP catalyst systems were synthesized: metal-intercalated ZrP
and metal-adsorbed ZrP, each involving Fe(II), Fe(III), Co(II), and Ni(II) cations. Fourier transform
infrared spectroscopy, X-ray powder diffraction, thermogravimetric analysis, and X-ray photoelectron
spectroscopy were used to characterize the composite materials and confirm the incorporation of the
metal cations either between the layers or on the surface of ZrP. Both types of metal-modified systems
were examined for their catalytic activity for the OER in 0.1 M KOH solution. All metal-modified
ZrP systems were active for the OER. Trends in activity are discussed as a function of the molar ratio
in relation to the two types of catalyst systems, resulting in overpotentials for metal-adsorbed ZrP
catalysts that were less than, or equal to, their metal-intercalated counterparts.
Keywords: zirconium phosphate; intercalation; ion exchange; catalyst support; oxygen evolution
reaction; electrocatalysis; water oxidation
1. Introduction
The renewable, sustainable production of hydrogen could have many benefits, displacing the
conventional fossil-based processes for its production and use in the fuels and chemical industry,
and potentially serving as an alternative energy vector given its high gravimetric energy density
and modularity of use [1]. Conventional hydrogen is produced through steam methane reforming,
but hydrogen produced by water electrolysis has the potential to be CO2 -emission free if the energy
required is provided by renewable energy sources. Technological improvements will be needed for the
cost-effectiveness of electrolysis to compete with current methods of hydrogen production [2,3]. Water
electrolysis is the decomposition of water through two half-reactions of water splitting, the oxygen
evolution reaction (OER) and the hydrogen evolution reaction (HER). The OER is important for many
renewable energy conversion technologies, such as photoelectrochemical cells, regenerative fuel cells,
and electrolyzers [1,4,5]. In acid systems, Ir and Ru-based oxides are common catalysts for the OER,
while in alkaline environments non-precious transition metal oxides based on Ni, Fe, and Co are
commonly employed [6–10]. In all cases, OER catalysts suffer from considerable overpotential losses,
limiting the efficiency of hydrogen generation by water electrolysis processes [11].
To overcome these challenges and thus to facilitate the economic viability of electrolysis,
the efficiencies of electrolyzer systems must be improved by addressing the kinetic overpotential
Catalysts 2017, 7, 132; doi:10.3390/catal7050132
www.mdpi.com/journal/catalysts
Catalysts 2017, 7, 132 2 of 14 Catalysts
2017,
7, 132
2 ofthe 14
To overcome these challenges and thus to facilitate the economic viability of electrolysis, efficiencies of electrolyzer systems must be improved by addressing the kinetic overpotential losses associated with turning over the OER and, ideally, using catalysts based on Earth‐abundant elements. losses
associated with turning over the OER and, ideally, using catalysts based on Earth-abundant
Recent studies have focused on reducing the amount of precious metals in OER catalysis, using more elements.
Recent studies have focused on reducing the amount of precious metals in OER catalysis,
Earth‐abundant transition metals in their oxide and/or hydroxide form to catalyze the reaction [12–
using more Earth-abundant transition metals in their oxide and/or hydroxide form to catalyze the
14]. However, as reported by recent experimental benchmarking efforts and supported by theoretical reaction
[12–14]. However, as reported by recent experimental benchmarking efforts and supported
calculations, overpotentials of state‐of‐the‐art catalysts have plateaued to the close
theoretical by
theoretical the calculations,
the overpotentials
of state-of-the-art
catalysts
haveclose plateaued
to the
minimum overpotential for the OER, predicted for conventional materials, including those based on theoretical minimum overpotential for the OER, predicted for conventional materials, including those
precious metals [15,16]. Significant research has been conducted to improve electrocatalysts through based
on precious metals [15,16]. Significant research has been conducted to improve electrocatalysts
a number of general strategies, including increasing the number of active sites, increasing the intrinsic through a number of general strategies, including increasing the number of active sites, increasing the
activity of each active site, and/or supporting the active material onto supports that engender intrinsic activity of each active site, and/or supporting the active material onto supports that engender
improved performance [17–21]. In particular, catalyst supports have shown to play an important role improved performance [17–21]. In particular, catalyst supports have shown to play an important role
in the performance and stability of catalysts for many different reactions [22–24]. in the performance and stability of catalysts for many different reactions [22–24].
In this study, the use of the zirconium phosphate (ZrP) type of layered inorganic compounds, as In this study, the use of the zirconium phosphate (ZrP) type of layered inorganic compounds,
a support for water oxidation catalysts, is examined. The most extensively studied phase of ZrP is as a support for water oxidation catalysts, is examined. The most extensively studied phase of ZrP
zirconium bis(monohydrogen orthophosphate) monohydrate (Zr(HPO4)2∙H2O, α‐ZrP), a layered is zirconium bis(monohydrogen orthophosphate) monohydrate (Zr(HPO4 )2 ·H2 O, α-ZrP), a layered
compound with an interlayer distance of 7.6 Å and a layer thickness of 6.6 Å (Figure 1A) [25]. α‐ZrP compound with an interlayer distance of 7.6 Å and a layer thickness of 6.6 Å (Figure 1A) [25]. α-ZrP
has a structure in which the Zr atoms in each layer align nearly to a plane with bridging phosphate has a structure in which the Zr atoms in each layer align nearly to a plane with bridging phosphate
groups 5.3 Å apart, which are located alternately above and below the metal atom plane [25]. Three groups 5.3 Å apart, which are located alternately above and below the metal atom plane [25]. Three
oxygen atoms of the phosphate group bond to three different Zr4+ ions; six O atoms from different oxygen atoms of the phosphate group bond to three different Zr4+ ions; six O atoms from different
phosphate groups coordinate the Zr atoms forming an octahedral coordination symmetry around phosphate groups coordinate the Zr atoms forming an octahedral coordination symmetry around
each Zr atom. These bridging phosphate groups provide an ion exchange site where the proton can each Zr atom. These bridging phosphate groups provide an ion exchange site where the proton
be exchanged with cations and other species. The above‐mentioned structure of α‐ZrP creates a small can be exchanged with cations and other species. The above-mentioned structure of α-ZrP creates
molecule‐sized cavity in the interlayer region with a diameter of 2.61 Å, which is occupied by a water a small molecule-sized cavity in the interlayer region with a diameter of 2.61 Å, which is occupied
molecule [25]. For α‐ZrP, the direct intercalation of small cations is possible if they are smaller than by a water molecule [25]. For α-ZrP, the direct intercalation of small cations is possible if they are
2.61 Å, but for larger cations and molecules, intercalation is not significant and/or these species are smaller than 2.61 Å, but for larger cations and molecules, intercalation is not significant and/or these
exchanged at very slow rates [26–28]. A direct intercalation method that can circumvent this problem species are exchanged at very slow rates [26–28]. A direct intercalation method that can circumvent
involves a highly hydrated phase of zirconium phosphate, Zr(HPO4)2∙6H2O, θ‐ZrP [29]. θ‐ZrP this problem involves a highly hydrated phase of zirconium phosphate, Zr(HPO4 )2 ·6H2 O, θ-ZrP [29].
maintains the α‐ZrP‐type layered structure (Figure 1B), but has an interlayer distance of 10.4 Å and θ-ZrP maintains the α-ZrP-type layered structure (Figure 1B), but has an interlayer distance of 10.4 Å
has six water molecules per formula unit, in contrast with α‐ZrP that only has one [30]. Figure 1C and has six water molecules per formula unit, in contrast with α-ZrP that only has one [30]. Figure 1C
shows transmission electron micrographs of α‐ZrP nanoparticles with the characteristic hexagonal shows transmission electron micrographs of α-ZrP nanoparticles with the characteristic hexagonal
nano‐platelet structure with an average diameter and thickness of ~140 and ~15 nm, respectively. nano-platelet structure with an average diameter and thickness of ~140 and ~15 nm, respectively.
Figure 1. (A) The structure of α‐ZrP; (B) the structure of θ‐ZrP; and (C) TEM micrographs of α‐ZrP Figure
1. (A) The structure of α-ZrP; (B) the structure of θ-ZrP; and (C) TEM micrographs of α-ZrP
nanoparticles where the top image shows the characteristic hexagonal shape and the bottom image nanoparticles where the top image shows the characteristic hexagonal shape and the bottom image
shows the thickness of an α‐ZrP nanoparticle. shows
the thickness of an α-ZrP nanoparticle.
θ‐ZrP can be directly ion‐exchanged with large metal complexes, producing intercalated phases θ-ZrP can be directly ion-exchanged with large metal complexes, producing intercalated phases
useful for artificial photosynthesis schemes, amperometric biosensors, vapochromic materials, useful for artificial photosynthesis schemes, amperometric biosensors, vapochromic materials, catalysis,
Catalysts 2017, 7, 132
3 of 14
and drug delivery applications [31–35]. Herein, the layered structure of ZrP is leveraged to support
active catalysts for the OER through the ion exchange of transition metal cations. Intercalation of
various metals of divalent and trivalent character (Fe(II), Fe(III), Co(II), Ni(II)) were performed on
θ-ZrP at various metal to θ-ZrP molar ratios. For comparison, we also investigate the same transition
metal systems adsorbed onto the outer surface of α-ZrP, achieved using a similar synthesis procedure,
however, due to the much tighter interlayer spacing of α-ZrP compared to θ-ZrP, intercalation is not
readily achieved. These two types of metal-modified ZrP, metal-intercalated ZrP and metal-adsorbed
ZrP, are studied herein for their physical and chemical properties, including as OER catalysts. These
oxygen evolution electrocatalyst materials were thoroughly characterized by Fourier transform infrared
spectroscopy (FTIR), thermogravimetric analysis (TGA), X-ray powder diffraction (XRPD), X-ray
photoelectron spectroscopy (XPS), and cyclic voltammetry. For the catalyst systems studied here in
alkaline electrolyte, we generally find that the metal-adsorbed ZrP catalysts are more active than their
intercalated counterparts, e.g., a metal-adsorbed Fe(II)-ZrP at a 10:1 Fe(II):ZrP molar ratio that requires
an overpotential of 490 mV at 10 mA/cm2 , while its metal-intercalated counterpart at the same molar
ratio requires an overpotential of 520 mV at 10 mA/cm2 .
2. Results and Discussion
2.1. FTIR and TGA Analysis of Interlayer and Surface Modification of ZrP
Each metal-modified (metal-intercalated and metal-adsorbed) ZrP sample was created through
suspensions of θ-ZrP and α-ZrP in deionized water mixed with Fe(II), Fe(III), Co(II), and Ni(II)
metal salts. The loading level was controlled by varying the metal:ZrP (M:ZrP) molar ratio used
in the suspension over a wide range (1:20–10:1), and the materials are referenced according to
this concentration ratio throughout the article. Figure 2 shows the IR spectra of pure α-ZrP,
metal-intercalated ZrP, and metal-adsorbed ZrP. The four characteristic α-ZrP bands associated with
lattice water were observed in the pure material. Three of them are sharp bands at 3592 cm−1 ,
3510 cm−1 , and 1616 cm−1 , while the fourth is a broad band at ca. 3141 cm−1 [36]. For the
metal-intercalated samples, the two sharp bands at 3592 cm−1 and 3510 cm−1 are not observed,
while the other two bands have reduced intensity compared to pure α-ZrP. When intercalation occurs,
the intercalant species will displace interlayer water molecules. For this reason, bands associated
with these water vibrational modes will show reduced relative intensity in the intercalated materials;
therefore, the IR data shown in Figure 2A is consistent with intercalation of the metal species into
ZrP interlayers, as also indicated by the XRPD data (vide infra). In contrast, metal-adsorbed samples
show very similar spectra to that of α-ZrP, with the sharp bands present at 3592 cm−1 and 3510 cm−1 ,
indicating negligible intercalation, as expected.
The characteristic orthophosphate group vibrations of ZrP are observed in the region of ca.
1057–966 cm−1 for all samples [36]. Metal-intercalated samples show a diminished relative intensity
of the shoulder at the bottom left part of the orthophosphate group vibrations at ca. 1050 cm−1 ,
attributed to the vibration of the exchangeable proton of the phosphate group, which is lost when the
proton is exchanged by intercalation via ion exchange with other species [36]. This is observed for all
metal-intercalated systems, another indication of successful intercalation. For metal-adsorbed samples,
this vibration is still present indicating, once again, that no intercalation is observed and, for clarity,
Figure S1 shows the characteristic 3592 and 3510 cm−1 peaks for adsorbed Fe(III)-ZrP.
Catalysts 2017, 7, 132
Catalysts 2017, 7, 132 4 of 14
4 of 14 A
B
Ni(II)
Ni(II)
Co(II)
Co(II)
Fe(III)
Fe(III)
Fe(II)
Fe(II)
ZrP
ZrP
3592 cm-1 3510 cm-1 3141 cm-1
3750
3000
3592 cm-1 3510 cm-1 3141 cm-1
1616 cm-1
2250
1500
Wavenumber (cm-1)
750
3750
3000
1616 cm-1
2250
1500
Wavenumber (cm-1)
750
Figure 2. FTIR spectra of (A) intercalated and (B) adsorbed Fe(II), Fe(III), Co(II), and Ni(II) at 10:1 Figure
2. FTIR spectra of (A) intercalated and (B) adsorbed Fe(II), Fe(III), Co(II), and Ni(II) at 10:1
M:ZrP ratio. M:ZrP ratio.
TGA was performed to further corroborate that intercalation had occurred for the metal‐
TGA was performed to further corroborate that intercalation had occurred for the
intercalated systems and that the metal‐adsorbed catalyst had retained α‐ZrP characteristics. In the metal-intercalated systems and that the metal-adsorbed catalyst had retained α-ZrP characteristics.
case of pure α‐ZrP (Figure 3) two weight losses are observed, dehydration of water within the In the case of pure α-ZrP (Figure 3) two weight losses are observed, dehydration of water within the
interlayer below 150 °C, followed by condensation to zirconium pyrophosphate at ~500 °C, leading interlayer below 150 ◦ C, followed by condensation to zirconium pyrophosphate at ~500 ◦ C, leading
to a total weight loss of ~13%, in agreement to previous literature results [37]. All metal‐intercalated to a total weight loss of ~13%, in agreement to previous literature results [37]. All metal-intercalated
systems (Figure 3A) showed significant deviation from pure α‐ZrP, especially within the temperature systems (Figure 3A) showed significant deviation from pure α-ZrP, especially within the temperature
range that pyrophosphate is formed. Based on thermograms for pure α‐ZrP as a reference, two range that pyrophosphate is formed. Based on thermograms for pure α-ZrP as a reference, two regions
regions are observed for Fe(II)‐intercalated systems—the dehydration of interlayer water are observed for Fe(II)-intercalated systems—the dehydration of interlayer water corresponding to
corresponding to an 8.7% weight loss at 76 °C, followed by a gradual shoulder decrease in weight an 8.7% weight loss at 76 ◦ C, followed by a gradual shoulder decrease in weight loss of 4% up to
loss of 4% up to 200 °C. Fe(III)‐intercalated systems show a 5% weight loss followed by a 3% weight 200 ◦ C. Fe(III)-intercalated systems show a 5% weight loss followed by a 3% weight loss for interlayer
loss for interlayer water dehydration and phosphate condensation, respectively. Similar results were water dehydration and phosphate condensation, respectively. Similar results were found to those
found to those observed in previous studies for TGA thermograms involving intercalated Co(II) and observed in previous studies for TGA thermograms involving intercalated Co(II) and Ni(II) ZrP
Ni(II) ZrP systems [38]. Both Co(II) and Ni(II)‐ZrP lose interlayer water below 150 °C for weight losses systems [38]. Both Co(II) and Ni(II)-ZrP lose interlayer water below 150 ◦ C for weight losses of ~12%
of ~12% and 10%, respectively. Since divalent and trivalent transition metal cations are highly and 10%, respectively. Since divalent and trivalent transition metal cations are highly hydrated in their
hydrated in their intercalated form [39]; previously‐reported reflectance and Extended X‐ray intercalated form [39]; previously-reported reflectance and Extended X-ray Absorption Fine Structure
Absorption Fine Structure (EXAFS) spectra show that Co(II)‐ and Ni(II)‐exchanged ZrP at room (EXAFS) spectra show that Co(II)- and Ni(II)-exchanged ZrP at room temperature are octahedrally
temperature are octahedrally coordinated with water [40,41]. The 3% weight loss between 200 and coordinated with water [40,41]. The 3% weight loss between 200 and 300 ◦ C for Co(II)-ZrP corresponds
300 °C for Co(II)‐ZrP corresponds to loss of metal coordinated waters, and the loss of structural water to loss of metal coordinated waters, and the loss of structural water due to decomposition of the
due to decomposition of the phosphate groups is evident between temperatures of 350 to 500 °C [38]. phosphate groups is evident between temperatures of 350 to 500 ◦ C [38]. For Ni(II)-intercalated
For Ni(II)‐intercalated samples, the loss of coordinated waters between 250 °C and 300 °C results in samples, the loss of coordinated waters between 250 ◦ C and 300 ◦ C results in a 5% weight loss,
a 5% weight loss, and decomposition of the phosphate groups to form pyrophosphate occurs between and decomposition of the phosphate groups to form pyrophosphate occurs between 350 ◦ C and
350 °C and 500 °C [38,42]. Ni(II)‐ZrP samples are more stable than Co(II)‐ZrP, as expected due to their 500 ◦ C [38,42]. Ni(II)-ZrP samples are more stable than Co(II)-ZrP, as expected due to their higher
higher ligand field stabilization energy [43]. Conversely, for all adsorbed cases (Figure 3B), their ligand field stabilization energy [43]. Conversely, for all adsorbed cases (Figure 3B), their respective
respective thermograms were similar to α‐ZrP indicating that no major modification of the interlayer thermograms were similar to α-ZrP indicating that no major modification of the interlayer had occurred
had occurred except for a reduction in weight loss for pyrophosphate, showing that the surface except for a reduction in weight loss for pyrophosphate, showing that the surface phosphates had
phosphates had undergone modification by the metal cations. All FTIR spectra and TGA undergone modification by the metal cations. All FTIR spectra and TGA thermograms are within
thermograms are within expected results for our ZrP systems and further substantiate that expected results for our ZrP systems and further substantiate that intercalated and adsorbed systems
intercalated and adsorbed systems were created as planned. were created as planned.
Catalysts 2017, 7, 132
Catalysts 2017, 7, 132 A
0.4
ZrP
Fe(II)
Co(II)
95
90
0.3
Fe(III)
Ni(II)
0.2
85
0.1
80
0.0
75
0
200
400
600
800
1000
Temperature (°C)
B
Weight (%)
100
ZrP
Fe(II)
Co(II)
95
90
0.2
Fe(III)
Ni(II)
0.1
85
0.0
80
0
200
400
600
Temperature (°C)
800
1000
Deriv. Weight Change (%/°C)
Weight (%)
100
Deriv. Weight Change (%/°C)
5 of 14
5 of 14 Figure 3. TGA thermograms of (A) intercalated and (B) adsorbed Fe(II), Fe(III), Co(II), and Ni(II) at a Figure 3. TGA thermograms of (A) intercalated and (B) adsorbed Fe(II), Fe(III), Co(II), and Ni(II) at
10:1 Solid lines
lines and dashed correspond to normalized weight and derivative
the first a 10:1M:ZrP M:ZrP ratio. ratio. Solid
and
dashed
lineslines correspond
to normalized
weight and
the first
derivative of weight change with respect to temperature, respectively. of weight change with respect to temperature, respectively.
2.2. XRPD Interlayer Expansion Analysis of Metal‐Modified ZrP 2.2. XRPD Interlayer Expansion Analysis of Metal-Modified ZrP
To assess possible interlayer expansion of ZrP sheets as a function of transition metal and cation To assess possible interlayer expansion of ZrP sheets as a function of transition metal and cation
valence, X‐ray powder diffraction (XRPD) measurements were performed for dried metal‐modified valence, X-ray powder diffraction (XRPD) measurements were performed for dried metal-modified
ZrP ZrP systems. systems. If Ifpure pure θ‐ZrP θ-ZrP is is dried, dried, dehydration dehydration converts converts it it to to α‐ZrP; α-ZrP; therefore, therefore, all all samples samples are are
◦
referenced to the α‐ZrP (002) diffraction peak at 2θ = 11.5°, corresponding to an interlayer distance referenced to the α-ZrP (002) diffraction peak at 2θ = 11.5 , corresponding to an interlayer distance
of 7.6 Å [44]. As described in the literature, an intercalation reaction can occur in a stepwise fashion of 7.6 Å [44]. As described in the literature, an intercalation reaction can occur in a stepwise fashion
depending on the intercalation energy threshold of the intercalants [45]. During this process, one of depending on the intercalation energy threshold of the intercalants [45]. During this process, one of
three possible patterns can be observed by XRPD for metal‐modified ZrP; either (i) a pattern with a three possible patterns can be observed by XRPD for metal-modified ZrP; either (i) a pattern with
peak corresponding to a larger interlayer spacing at lower 2θ values than 11.5° indicating that metal a peak corresponding to a larger interlayer spacing at lower 2θ values than 11.5◦ indicating that metal
cations were introduced
introduced the interlayer; (ii) a with
pattern with two distinct atpeaks—one at cations were
intointo the interlayer;
(ii) a pattern
two distinct
peaks—one
approximately
◦
approximately 2θ = 11.5° and one that appears at lower 2θ values than the reference peak indicating 2θ = 11.5 and one that appears at lower 2θ values than the reference peak indicating that a mixed
that a mixed phase is present; and (iii) a pattern with no change in the reference peak, indicating that phase is present; and (iii) a pattern with no change in the reference peak, indicating that the cation
the cation species is the outer the layered rather than being species
is adsorbed
onadsorbed the outeron surface
of the surface layered of structure
ratherstructure than being
intercalated
[44].
intercalated [44]. Below, the XRPD analysis for all metal‐modified ZrP systems is presented. Below, the XRPD analysis for all metal-modified ZrP systems is presented.
Figure 4A shows the XRPD of pure α‐ZrP and metal‐intercalated ZrP for Fe(II), Fe(III), Co(II), Figure 4A shows the XRPD of pure α-ZrP and metal-intercalated ZrP for Fe(II), Fe(III), Co(II), and
and Ni(II), at M:ZrP ranging from A stepwise is expected as a Ni(II), at M:ZrP
molarmolar ratiosratios ranging
from 1:20
to1:20 10:1.to A10:1. stepwise
process process is expected
as a function
function of intercalant solution molarity; the intercalation reaction initiates from the edges of the of intercalant solution molarity; the intercalation reaction initiates from the edges of the particle
particle and proceeds by diffusion of the metal cations towards the interior of the interlayer sheets and proceeds by diffusion of the metal cations towards the interior of the interlayer sheets [37].
[37]. The diffraction patterns for all four metal cation systems show a shift of the first diffraction peak The diffraction patterns for all four metal cation systems show a shift of the first diffraction peak
towards lower 2θ values compared to the (002) peak of pure α‐ZrP, indicating that new intercalated towards lower 2θ values compared to the (002) peak of pure α-ZrP, indicating that new intercalated
phases with expanded interlayer distance (>7.6 Å) were obtained, even for the lowest M:ZrP molar phases with expanded interlayer distance (>7.6 Å) were obtained, even for the lowest M:ZrP molar
ratio ratio of of 1:20. 1:20. Increasing Increasing the the M:ZrP M:ZrP molar molar ratios ratios to to 1:10 1:10 and and 1:3 1:3 results results in in further further broadening broadening and and
shifting of the peak across all samples, indicating a more mixed phase is present, and that the layered shifting of the peak across all samples, indicating a more mixed phase is present, and that the layered
structure has not achieved its maximum cation loading within the interlayer. At the highest loadings structure has not achieved its maximum cation loading within the interlayer. At the highest loadings
(i.e., 1:1 toto 10:1
10:1 molar ratios), original 2θ ◦= has
11.5° has disappeared, with a new peak (i.e., 1:1
molar
ratios),
thethe original
peakpeak at 2θat = 11.5
disappeared,
with a new
peak
emerging
emerging at significantly of a2θ, a final value indicative interlayer
of the maximum at significantly
lower valueslower of 2θ,values reaching
finalreaching value indicative
of the maximum
distance
interlayer distance for that particular metal cation intercalated within ZrP. Figure S2 shows for that particular metal cation intercalated within ZrP. Figure S2 shows additional data regarding
the
additional data regarding the (020) and (312 reflections of these catalyst systems, further confirming that the intercalated products for the metal‐intercalated systems retained the layered structure of ZrP. Catalysts 2017, 7, 132
6 of 14
Catalysts 2017, 7, 132 (020) and (312) reflections
6 of 14 of these catalyst systems, further confirming that the intercalated products
for the metal-intercalated systems retained the layered structure of ZrP.
As expected for
for anan ion‐exchange reaction, Figure 4B shows that intercalated with As expected
ion-exchange
reaction,
Figure
4B shows
that intercalated
samplessamples with divalent
divalent cations (i.e., Fe(II)) underwent a much larger interlayer spacing transformation than trivalent cations (i.e., Fe(II)) underwent a much larger interlayer spacing transformation than trivalent cations
cations (i.e., Fe(III)), an average of 2.0 0.6 Å, respectively. intercalation, (i.e., Fe(III)),
with anwith average
increaseincrease of 2.0 and
0.6and Å, respectively.
DuringDuring intercalation,
higher
higher valence species induce a strong electrostatic interaction with the anionic interlayers, which valence species induce a strong electrostatic interaction with the anionic interlayers, which mitigates
mitigates interlayer expansion [46]. Figure 4B also shows that, for the case of the metal‐adsorbed ZrP interlayer expansion [46]. Figure 4B also shows that, for the case of the metal-adsorbed ZrP systems,
systems, their respective interlayer spacing were unchanged and remained equal to that of α‐ZrP (7.6 their respective interlayer spacing were unchanged and remained equal to that of α-ZrP (7.6 Å) at
Å) at 1:1–1:10 conclusively showing these particular materials, metal is 1:1–1:10
molar molar ratios, ratios, conclusively
showing
that, forthat, thesefor particular
materials,
the metalthe is adsorbed
adsorbed on the surface and not intercalated. Figure shows the for
XRPD for these systems, on the surface
and
not intercalated.
Figure S3 shows
theS3 XRPD
data
thesedata systems,
revealing
that
revealing that the adsorbed species have an identical XRPD pattern to pure α‐ZrP confirming that the adsorbed species have an identical XRPD pattern to pure α-ZrP confirming that the characteristic
the characteristic α‐like layered structure is still intact for the metal‐adsorbed system. As the catalysts α-like layered structure is still intact for the metal-adsorbed system. As the catalysts were not processed
were not processed further (e.g., with heat treatments), the form of the metal resulting on the surface further (e.g., with heat treatments), the form of the metal resulting on the surface of ZrP is amorphous
of ZrP is amorphous and, as expected, no diffraction corresponding to those metals is observed by and, as expected, no diffraction corresponding to those metals is observed by XRPD. We note that it is
XRPD. We note that it is possible that the metal‐intercalated ZrP systems might also contain adsorbed possible that the metal-intercalated ZrP systems might also contain adsorbed metals on their surfaces
metals on their surfaces (that are also not observable by XRPD) in addition to those within the layers. (that are also not observable by XRPD) in addition to those within the layers.
B
A
Fe(III)
Fe(II)
Ni(II)
Co(II)
10.0
Fe(II)
Ni(II)
Co(II) 9.8
Fe(III)
9.6
Arbitrary
IntensityIntensity
(a.u)
9.2
9.0
8.8
8.6
8.4
8.2
Interlayer Distance (Å)
9.4
8.0
α-ZrP
α-ZrP
α-ZrP
7.8
α-ZrP
7.6
10
2θ (°)
12
8
10
2θ (°)
12
8
10
2θ (°)
12
8
10
2θ (°)
12
10:1
5:1
3:1
1:1
1:3
1:5
1:10
1:20
8
Molarity Ratio (Metal:ZrP)
Figure (A) XRPD
XRPD patterns
patterns Fe(II), Fe(III), Co(II), Ni(II)‐intercalated at top
(from top to Figure 4. 4. (A)
forfor Fe(II),
Fe(III),
Co(II),
and and Ni(II)-intercalated
ZrP atZrP (from
to bottom)
bottom) 10:1, 5:1, 3:1, 1:1, 1:3, 1:5, 1:10, and 1:20 M:ZrP molar ratios. The bottom diffraction pattern in 10:1, 5:1, 3:1, 1:1, 1:3, 1:5, 1:10, and 1:20 M:ZrP molar ratios. The bottom diffraction pattern in all
molar for the all frames is that of pure α‐ZrP. (B) Interlayer distance as a function of M:ZrP frames
is that
of pure
α-ZrP.
(B) Interlayer
distance
as a function
of M:ZrP
molar ratio
forratio the various
various metal‐intercalated ZrP materials. Metal‐adsorbed systems are represented as dashed metal-intercalated
ZrP materials.
Metal-adsorbed
systems are
represented
as dashed lines
which lines have
which have an exact interlayer spacing as pure α‐ZrP indicating that metal intercalation did not occur. an exact interlayer spacing as pure α-ZrP indicating that metal intercalation did not occur.
2.3. XPS Analysis of Metal Loading and Chemical Composition of Metal‐Modified ZrP 2.3. XPS Analysis of Metal Loading and Chemical Composition of Metal-Modified ZrP
To quantify the
the relative amounts of cations the metal‐modified ZrP and
systems, and to To quantify
relative
amounts
of cations
for thefor metal-modified
ZrP systems,
to investigate
investigate the chemical nature of the metal, XPS spectra were obtained for metal‐modified catalysts the chemical nature of the metal, XPS spectra were obtained for metal-modified catalysts at 10:1 M:ZrP
at 10:1 M:ZrP molar ratios for all four metals and at 1:1 to 10:1 molar ratios for the Fe(II)‐ZrP system. molar ratios for all four metals and at 1:1 to 10:1 molar ratios for the Fe(II)-ZrP system. Figure 5 shows
Figure 5 shows XPS atomic concentrations of intercalated and adsorbed M(II/III):ZrP at high loadings. XPS atomic concentrations of intercalated and adsorbed M(II/III):ZrP at high loadings. As expected,
As expected, due to the uptake of metal cations within the much larger area of the interlayers of ZrP due to the uptake of metal cations within the much larger area of the interlayers of ZrP rather than
rather than on inthe in case,
the adsorbed case, XPS high‐resolution show that solely on
thesolely surface
thesurface adsorbed
XPS high-resolution
scans show thatscans intercalated
ZrP
intercalated ZrP systems have higher atomic metal content when compared to adsorbed systems at systems have higher atomic metal content when compared to adsorbed systems at similar M:ZrP ratios.
similar M:ZrP ratios. As depicted in Figure 6, the characteristic peaks for Zr, P, and the respective As depicted in Figure 6, the characteristic peaks for Zr, P, and the respective metals were present for
metals were present for all samples. all samples.
The XPS spectra of Fe(II) samples in Figure 5A show the Fe 2p3/2 and Fe 2p1/2 peaks for the intercalated samples at 712.4 eV and 725.8 eV, respectively, and for the adsorbed samples at 713.0 eV and 726.6 eV, respectively. The peak positions of Fe(II) 2p3/2 and 2p1/2 peaks in oxide materials are expected at approximately 709.0 eV and 723.0 eV, respectively [47]. Figure 6A shows that for both the Catalysts 2017, 7, 132
7 of 14
The XPS spectra of Fe(II) samples in Figure 5A show the Fe 2p3/2 and Fe 2p1/2 peaks for the
intercalated samples at 712.4 eV and 725.8 eV, respectively, and for the adsorbed samples at 713.0 eV
Catalysts 2017, 7, 132 7 of 14 and 726.6 eV, respectively. The peak positions of Fe(II) 2p3/2 and 2p1/2 peaks in oxide materials are
expected at approximately 709.0 eV and 723.0 eV, respectively [47]. Figure 6A shows that for both the
Fe(II)‐intercalated and the Fe(II)‐adsorbed systems, higher binding energies are observed, which Fe(II)-intercalated and the Fe(II)-adsorbed systems, higher binding energies are observed, which might
might correspond to partial oxidation of the Fe(II) species upon ZrP incorporation due to interactions correspond to partial oxidation of the Fe(II) species upon ZrP incorporation due to interactions with
with the phosphate groups; the Fe 2p3/2 peak in FePO4 is expected at 712.8 eV [48]. On the other hand, the phosphate groups; the Fe 2p3/2 peak in FePO4 is expected at 712.8 eV [48]. On the other hand,
the Fe(III)‐intercalated sample, Figure 5A shows the Fe 2p
3/2 peak at 711.6 eV, which is more consistent the Fe(III)-intercalated sample, Figure 5A shows the Fe 2p3/2 peak at 711.6 eV, which is more consistent
with previously‐reported values expected for Fe(III) oxides, which generally exhibit the Fe 2p
3/2 peak with previously-reported values expected for Fe(III) oxides, which generally exhibit the Fe 2p3/2 peak
between 710.6 and 711.2 eV [47]. between 710.6 and 711.2 eV [47].
A
B
0.8
0.45
Adsorbed
0.7
M:Zr (Atomic Concentration)
Fe(II):Zr (Atomic Concentration)
Intercalated
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.40
Intercalated
Adsorbed
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
1:1
3:1
5:1
10:1
Fe(II):ZrP (Molarity Ratio)
Fe(III)
Co(II)
Ni(II)
10:1 M:ZrP (Molarity Ratio)
Figure 5. 5. (A) (A) Atomic Atomic concentration concentration Fe:Zr Fe:Zr ratios ratios for
Fe(II):ZrP molar molar ratio ratio
Figure for metal-modified
metal‐modified 1:1–10:1
1:1‐10:1 Fe(II):ZrP catalysts.
(B)
Atomic
concentration
M:Zr
ratios
for
Fe(III),
Co(II),
and
Ni(II)
metal-modified
10:1
M:ZrP
catalysts. (B) Atomic concentration M:Zr ratios for Fe(III), Co(II), and Ni(II) metal‐modified 10:1 molar ratio catalysts.
M:ZrP molar ratio catalysts. Deconvolution of the XPS spectra for Co(II) and Ni(II) is challenging due to the small shifts in Deconvolution of the XPS spectra for Co(II) and Ni(II) is challenging due to the small shifts in the
the binding energy for the (II) and (III) metal oxidation states and the broad satellite peaks [49,50]. binding energy for the (II) and (III) metal oxidation states and the broad satellite peaks [49,50]. For the
For the Co(II) Figure
sample, a spectrum
Co 2p XPS spectrum with 2ppeaks
3/2 and 2p1/2characteristic
peaks with Co(II)
sample,
6AFigure shows 6A a Coshows 2p XPS
with
2p3/2 and
2p1/2
with
characteristic similar to those seen in zirconium
other cobalt‐based phosphate satellite peaks,satellite similar topeaks, those seen
in other
cobalt-based
phosphate zirconium catalysts, but
at higher
catalysts, but at higher binding energies, suggesting a partial oxidation of the Co species, containing binding energies, suggesting a partial oxidation of the Co species, containing both Co(II) and Co(III)
both Co(II) and Co(III) characteristics [51].
For the Ni(II)‐ZrP, Figure 6A shows a Ni 2p XPS spectrum characteristics [51]. For the Ni(II)-ZrP, Figure
6A shows a Ni 2p XPS spectrum with 2p3/2 and 2p1/2
with 2p
3/2 and 2p
1/2 peaks with satellite peaks, as well. The positions of the 2p
3/2 and 2p
1/2 peaks lie in peaks with
satellite
peaks, as well. The positions of the 2p3/2 and 2p1/2 peaks
lie in between
that of
between that of nickel oxide and nickel phosphate potentially elucidating that Ni cations are strongly nickel oxide and nickel phosphate potentially elucidating that Ni cations are strongly interacting with
interacting with both oxygen and phosphorus atoms in the ZrP [50,52]. both oxygen and phosphorus atoms in the ZrP [50,52].
Figure 6B shows the Zr 3d
5/2 and 3d
peaks and the P 2s peak for both intercalated and adsorbed Figure 6B shows the Zr 3d5/2
and 3d3/2
peaks and the P 2s peak for both intercalated and adsorbed
3/2
samples, for the 10:1 Fe(II)‐ZrP sample. In addition, Figure 6C shows the P 2p
and 2p
1/2 peaks which samples, for the 10:1 Fe(II)-ZrP sample. In addition, Figure 6C shows the P3/22p
2p1/2 peaks
3/2 and
overlap and are, therefore, observed as a single, broad peak. For intercalated samples (blue traces), which overlap and are, therefore, observed as a single, broad peak. For intercalated samples (blue
the binding energies observed for Zr and P are similar to those previously reported. traces), the binding energies observed for Zr and P are similar to those previously reported.
Previous reports ofof XPS results modified systems that intercalants can Previous reports
XPS
results
forfor modified
ZrP ZrP systems
have have shownshown that intercalants
can modify
modify the polarization of both Zr and P atoms and, as a result, vary their respective binding energies the polarization of both Zr and P atoms and, as a result, vary their respective binding energies when
when compared to pristine α‐ZrP [53,54]. For the adsorbed 10:1 Fe(II)‐ZrP sample, two Zr species are compared to pristine α-ZrP [53,54]. For the adsorbed 10:1 Fe(II)-ZrP sample, two Zr species are
observed, a doublet with peaks of lower intensity, but higher binding energy (red traces), and another observed, a doublet with peaks of lower intensity, but higher binding energy (red traces), and another
doublet with binding energy as reported in the literature for α‐ZrP (black traces). The high‐resolution doublet with binding energy as reported in the literature for α-ZrP (black traces). The high-resolution
scans for the Zr 3d region in M:Zr samples show an increase in binding energy compared to that of scans for the Zr 3d region in M:Zr samples show an increase in binding energy compared to that of
α‐ZrP, which indicates an increase in the polarization of the Zr atoms in the adsorbed samples due α-ZrP, which indicates an increase in the polarization of the Zr atoms in the adsorbed samples due to
to interactions with the metal species. Similar results are seen for the P 2p
and 2p
1/2 binding energy interactions with the metal species. Similar results are seen for the P 2p3/23/2and
2p1/2
binding energy
region, where the polarization also produces a shift to increased binding energy compared to the values in α‐ZrP. The presence of this polarization on the Zr and P atoms is more pronounced on the metal‐adsorbed ZrP samples because the metal concentration on the surface of ZrP is higher than the metal‐intercalated ZrP and, hence, the surface sensitivity of XPS can detect this binding energy shift. Catalysts 2017, 7, 132
8 of 14
region, where the polarization also produces a shift to increased binding energy compared to the
values in α-ZrP. The presence of this polarization on the Zr and P atoms is more pronounced on the
Catalysts 2017, 7, 132 8 of 14 metal-adsorbed ZrP samples because the metal concentration on the surface of ZrP is higher than
the
metal-intercalated ZrP and, hence, the surface sensitivity of XPS can detect this binding energy shift.
While While the the XPS XPS measurements measurements help help to to inform inform the the chemical chemical state state of of the the metal‐modified metal-modified ZrP ZrP
systems, we note that changes are expected from their dry powder form to that in an electrochemical systems, we note that changes are expected from their dry powder form to that in an electrochemical
environment, held at the oxidative potentials needed for water oxidation in an alkaline solution. environment, held at the oxidative potentials needed for water oxidation in an alkaline solution.
A
Intensity (a.u.)
Fe 2p
Fe 2p
Fe(II)
730 720
B.E. (eV)
710
Fe(III)
Co 2p
730 720 710 810
B.E. (eV)
B
Co(II)
795
B.E. (eV)
Ni 2p
780 880
Ni(II)
870 860
B.E. (eV)
C
P 2p
Intensity (a.u.)
Intensitity (a.u.)
Zr 3d
195
190 185
B.E. (eV)
180
140
135
B.E. (eV)
130
Figure 6. (A) XPS spectra for Fe(II), Fe(III), Co(II), and Ni(II) ZrP systems. Blue and green curves in Figure 6. (A) XPS spectra for Fe(II), Fe(III), Co(II), and Ni(II) ZrP systems. Blue and green curves in
Fe(II) panel represent
represent M:ZrP intercalated and adsorbed respectively, while Fe(II) panel
thethe 10:110:1 M:ZrP
intercalated
and adsorbed
systems, systems, respectively,
while intercalated
intercalated 10:1 M:ZrP systems are represented in the Fe(III), Co(II), and Ni(II) panels. (B) XPS 10:1 M:ZrP systems are represented in the Fe(III), Co(II), and Ni(II) panels. (B) XPS spectra for Zr
3d
spectra for Zr 3d and (C) P 2p where blue and green curves represent intercalated and adsorbed and (C) P 2p where blue and green curves represent intercalated and adsorbed systems, respectively,
systems, respectively, from 10:1 Fe(II):ZrP. from 10:1 Fe(II):ZrP.
2.4. Electrochemical Studies of Metal‐ModifiedZrP 2.4. Electrochemical Studies of Metal-ModifiedZrP
To study the electrocatalytic activity of metal‐modified ZrP systems for the OER, cyclic To study the electrocatalytic activity of metal-modified ZrP systems for the OER, cyclic
voltammetry (CV) measurements were performed in a three‐electrode rotating disk electrode (RDE) voltammetry (CV) measurements were performed in a three-electrode rotating disk electrode (RDE)
configuration with a 0.1 M KOH electrolyte. To make the working electrode, the metal‐modified ZrP configuration with a 0.1 M KOH electrolyte. To make the working electrode, the metal-modified ZrP
catalysts were mixed with carbon black in a water‐based solution and deposited onto a glassy carbon catalysts were mixed with carbon black in a water-based solution and deposited onto a glassy carbon
disk. One figure of merit conventionally used to describe OER catalyst performance is the disk. One figure of merit conventionally used to describe OER catalyst performance is the overpotential
overpotential necessary to reach a current density 2 of 10 mA/cm2 [15]. Figures 7A,B show cyclic necessary to reach a current density of 10 mA/cm [15]. Figure 7A,B show cyclic voltammograms
voltammograms for the highest performance metal‐modified ZrP system with either Fe(II) adsorbed for the highest performance metal-modified ZrP system with either Fe(II) adsorbed or intercalated,
or intercalated, while the other systems can be found in Figure S4. For reference, pure α‐ZrP loaded while the other systems can be found in Figure S4. For reference, pure α-ZrP loaded onto carbon
2 even at onto carbon black shows only moderate activity for the OER, unable to achieve 10 mA/cm
black shows only moderate activity for the OER, unable to achieve 10 mA/cm2 even at high oxidative
high oxidative potentials, which is comparable to other Zr‐based systems [55]. Once metal cations are potentials, which is comparable to other Zr-based systems [55]. Once metal cations are ion exchanged
ion exchanged into and/or supported onto ZrP, an active OER catalyst is produced for both metal‐
into and/or supported onto ZrP, an active OER catalyst is produced for both metal-modified ZrP
modified ZrP systems at various M:ZrP ratios. Adsorbed Fe(II)‐ZrP shows a gradual increase in systems at various M:ZrP ratios. Adsorbed Fe(II)-ZrP shows a gradual increase in performance as the
performance as the M:ZrP concentration increases and similar results are observed for the M:ZrP concentration increases and similar results are observed for the intercalated case. As shown in
intercalated case. As shown in Figure 5, higher metal content is achieved at higher M:ZrP molar ratios Figure 5, higher metal content is achieved at higher M:ZrP molar ratios for both systems; therefore,
for both systems; therefore, it is likely that this gradual increase in activity can be attributed to higher it is likely that this gradual increase in activity can be attributed to higher atomic concentrations.
atomic concentrations. The adsorbed 10:1 Fe(II):ZrP molar ratio exhibits the highest OER The adsorbed 10:1 Fe(II):ZrP molar ratio exhibits the highest OER performance of all the systems
performance of all the systems investigated herein, requiring an overpotential of 490 mV at 10 investigated herein, requiring an overpotential of 490 mV at 10 mA/cm2 with a 0.55:1 Fe:Zr atomic
mA/cm2 with a 0.55:1 Fe:Zr atomic ratio, as determined by XPS. For comparison, the metal‐
ratio, as determined by XPS. For comparison, the metal-intercalated equivalent system of 10:1 Fe(II):
intercalated equivalent system of 10:1 Fe(II): ZrP (with a higher Fe:Zr atomic ratio of 0.63:1) requires a higher overpotential of 524 mV, suggesting that the metal intercalated within ZrP may not be electrochemically accessible for water oxidation. Figure 7C shows electrochemical data comparing the other metal‐modified ZrP systems. All metal‐modified systems are catalytically active, requiring between 0.5 and 0.7 V of overpotential to reach 10 mA/cm2, depending on the choice of the metal cation, the M:ZrP molar ratio used during Catalysts 2017, 7, 132
9 of 14
ZrP (with a higher Fe:Zr atomic ratio of 0.63:1) requires a higher overpotential of 524 mV, suggesting
that the metal intercalated within ZrP may not be electrochemically accessible for water oxidation.
Figure 7C shows electrochemical data comparing the other metal-modified ZrP systems.
Catalysts 2017, 7, 132 9 of 14 All metal-modified systems are catalytically active, requiring between 0.5 and 0.7 V of overpotential
to reach 10 mA/cm2 , depending on the choice of the metal cation, the M:ZrP molar ratio used
overpotentials are observed for the higher M:ZrP molar ratios, ascribed to higher metal loadings. For during synthesis, and whether the metal was intercalated into, or adsorbed onto, ZrP. In general,
the Fe(II):ZrP
and Ni(II):ZrP catalysts, intercalated systems consistently require lower
overpotentials
are observed
for thethe higher
M:ZrP molar
ratios,
ascribed to
higherhigher metal
overpotentials versus their adsorbed counterparts, e.g., by 34 mV and 88 mV, respectively, at a M:ZrP loadings. For the Fe(II):ZrP and Ni(II):ZrP catalysts, the intercalated systems consistently require
ratio of 10:1. A less pronounced difference in activity for the two types of metal‐modification was higher overpotentials versus their adsorbed counterparts, e.g., by 34 mV and 88 mV, respectively, at a
observed for ofthe cases of Fe(III):ZrP and Co(II):ZrP. Given that, in two
general, activities for the M:ZrP ratio
10:1.
A less
pronounced
difference
in activity
for the
typesOER of metal-modification
metal‐adsorbed ZrP catalysts are greater than or equal to those of their metal‐intercalated was observed for the cases of Fe(III):ZrP and Co(II):ZrP. Given that, in general, OER activities for the
counterparts at the loading, it is likely the toOER dominated by catalysis on the outer metal-adsorbed
ZrP same catalysts
are greater
than orthat equal
thoseis of
their metal-intercalated
counterparts
surface of the ZrP supported metal‐based systems rather than within the layers, which may be limited at the same loading, it is likely that the OER is dominated by catalysis on the outer surface of the ZrP
by mass transport. supported metal-based systems rather than within the layers, which may be limited by mass transport.
B
30
ZrP/CB
1:1
3:1
5:1
10:1
10:1 Int.
20
15
10
ZrP/CB
1:1
3:1
5:1
10:1
20
15
10
5
5
0
0
1.5
C
1.6
1.7
1.5 1.6 1.7 1.8 1.9
1.8
Evs RHE(V)
Evs RHE(V)
700
Ni(II)
Ni(II)
Fe(II)
Fe(II)
600
Overpotential (mV)
25
2
2
j (mA/cm )
25
j (mA/cm )
A
500
600
Fe(III)
Fe(III)
550
500
600
550
Co(II)
Co(II)
500
1:20
1:10
1:5
1:3
1:1
3:1
Molarity Ratio (Metal:ZrP)
5:1
10:1
Figure 7. (A) Cyclic voltammograms of adsorbed Fe‐ZrP. The best CV for intercalated Fe(II)‐ZrP is Figure 7. (A) Cyclic voltammograms of adsorbed Fe-ZrP. The best CV for intercalated Fe(II)-ZrP is
shown in pink. (B) Cyclic voltammograms of intercalated Fe‐ZrP. (C) Electrochemical performance shown in pink. (B) Cyclic voltammograms of intercalated Fe-ZrP. (C) Electrochemical performance
comparison of all four metal systems for adsorbed and intercalated species at 10 mA/cm
comparison of all four metal systems for adsorbed and intercalated species at 10 mA/cm22, except for , except for
2 . Solid and dashed lines represent intercalated and adsorbed
Ni(II), which was compared at 3 mA/cm
Ni(II), which was compared at 3 mA/cm2. Solid and dashed lines represent intercalated and adsorbed metal ZrP systems, respectively metal ZrP systems, respectively
While mechanism of OER catalysis remains unclear for either metal‐
While at atthis thisstage stagethe the
mechanism
of OER
catalysis
remains
unclear
for the either
the
intercalated ZrP or the ZrP, ZrP,
a further question involves metal-intercalated
ZrP
ormetal‐adsorbed the metal-adsorbed
a further
question
involvesconductivity, conductivity,as as the the
intercalation of metals within the interlayers could affect the conductivity of the ZrP nanoparticle intercalation of metals within the interlayers could affect the conductivity of the ZrP nanoparticle [56].
[56]. Furthermore, all systems, from Ni(II), exhibited diminishing stability with continued Furthermore,
all systems,
apart apart from Ni(II),
exhibited
diminishing
stability
with continued
cycling
cycling (Figure S4). Further work is aimed at enhancing the conductivity of the ZrP supports and (Figure S4). Further work is aimed at enhancing the conductivity of the ZrP supports and tailoring the
tailoring the surface and interlayer properties to improve stability. surface and interlayer properties to improve stability.
3. Conclusions Our findings indicate that ZrP can serve as a support for transition metal‐based OER catalysts. Increasing the M:ZrP molar ratio had a direct increase in metal content observed by XPS which correlated to an increase in activity for all metal systems. Consequently, as observed by electrochemical cyclic voltammetry, the metal species that were supported within ZrP required Catalysts 2017, 7, 132
10 of 14
3. Conclusion
Our findings indicate that ZrP can serve as a support for transition metal-based OER catalysts.
Increasing the M:ZrP molar ratio had a direct increase in metal content observed by XPS which
correlated to an increase in activity for all metal systems. Consequently, as observed by electrochemical
cyclic voltammetry, the metal species that were supported within ZrP required similar, or higher,
overpotentials to drive the OER when compared to their metal-adsorbed counterparts. This may result
from differences in mass transport and/or electronic conductivity in the two systems. This report on
metal-modified ZrP catalysts serves as a basis for developing improved OER catalyst systems, paving
the way for future work investigating various metal combinations or molecular species supported
onto, or into, ZrP for enhanced catalytic activity and stability.
4. Materials and Methods
4.1. Materials
All chemicals were obtained from commercial sources as analytical or reagent grade and used
as received. Zirconium oxychloride octahydrate (ZrOCl2 ·8H2 O, 98%), iron chloride tetrahydrate
(FeCl2 ·4H2 O, 98%), iron nitrate nonahydrate (Fe(NO3 )3 ·9H2 O, 98%), cobalt nitrate hexahydrate
(Co(NO3 )2 ·6H2 O, 98%), nickel nitrate hexahydrate (Ni(NO3 )2 ·6H2 O, 98.5%), and Nafion® 117 (5%)
were obtained from Sigma-Aldrich (St. Louis, MO, USA). Phosphoric acid (H3 PO4 , 85% v/v)
was obtained from Fisher Scientific (Hampton, NH, USA), glassy carbon rods (SigradurG, HTW
Hochtemperatur-Werkstoffe GmbH, 5 mm diameter) were processed by the Stanford University crystal
shop to prepare 4 mm long pieces with the top side polished to a surface root-mean-square (RMS)
roughness of less than 50 nm, and carbon black was purchased from Fuel Cell Store (College Station,
TX, USA, Vulcan XC-72).
4.2. θ-, α-, and Metal-Modified ZrP Synthesis
The synthesis for θ-ZrP was the same as reported by Kijima [57]. First, 100 mL of 6 M H3 PO4
was heated to 94 ◦ C under constant stirring, followed by the drop-wise addition of 100 mL of 0.05 M
ZrOCl2 ·8H2 O aqueous solution. The resulting mixture was refluxed with constant stirring at 94 ◦ C for
two days forming solid θ-ZrP particles suspended in aqueous solution. The solution was centrifuged
at 10,000 RPM for 20 min, followed by removing the supernatant and then washing the remaining solid
with water (18.2 MΩ·cm, Merck Millipore, Billerica, MA, USA). This process was repeated three times,
resulting in a gelatinous paste material. θ-ZrP was converted into α-ZrP by allowing θ-ZrP to dry
overnight under vacuum at room temperature. θ-ZrP was suspended in deionized water at various
M:ZrP molar ratios (10:1, 5:1, 3:1, 1:1, 1:3, 1:5, 1:10, and 1:20) with constant stirring at room temperature
for five days, producing a metal-intercalated ZrP catalyst at different loading levels, where “M” refers
to different metal cations studied: Fe(II), Fe(III), Co(II), and Ni(II). Each respective sample was filtered
and washed with water three times and set to dry in vacuum overnight. A similar procedure was
followed to produce the high molarity (1:1, 3:1, 5:1, and 10:1) ratios for the metal-modified α-ZrP with
a residence time of one day. The samples were centrifuged and washed, as mentioned above, and all
resulting powders were pulverized and sonicated to ease their dispersion in ink solutions.
4.3. Physical and Chemical Characterization (TEM, FTIR, TGA, XRPD, XPS)
Transmission electron microscopy (TEM) was performed by a FEI Tecnai (Thermo Fisher Scientific,
Inc. (Waltham, MA, USA)) operated at 200 kV. Vibrational spectroscopy data was collected from 4000
to 650 cm−1 using a Thermo Fisher Scientific, Inc. (Waltham, MA, USA) Nicolet iS50 FT-IR continuum
microscope. Before analysis, the samples were prepared in the following manner: for each sample,
a 2 µL aqueous suspension was drop-casted onto an International Crystal Laboratories (Garfield, NJ,
USA) Real Crystal® KBr IR card. Samples were dried on the IR card with argon gas for ca. 2 min.
Thermogravimetric experiments (TGA Q500 TA Instruments, New Castle, DE, USA) were performed
Catalysts 2017, 7, 132
11 of 14
at a ramp rate of 5 ◦ C min−1 under a flow of 90 mL/min air and 10 mL/min nitrogen up to 1000 ◦ C.
The lattice spacing and crystallinity of the catalysts were investigated using X-ray powder diffraction
(Philips PANanalytical, Almelo, The Netherlands, X’Pert Pro) with Cu Kα radiation (λ = 1.54184 Å).
The interlayer distance of the intercalated products was obtained from Bragg’s Law (nλ = 2dhkl sin θ)
using the angle at which the first-order diffraction peak appears in the diffractograms, where λ is the
wavelength of the X-ray source, dhkl is the interlayer distance between planes in the unit cell and θ is
the diffraction angle. The chemical state and atomic concentration of M and Zr were examined using
high-resolution XPS (PHI 5000 VersaProbe, Physical Electronics, Enzo, Chigasaki, Japan) with an Al
Kα source. All spectra were calibrated to the adventitious carbon 1s peak at 284.8 eV and fitted using
a Shirley background and constraints for doublets, such as full width at half maximum, peak areas,
and peak splitting, were taken into consideration for each respective transition metal and nonmetal.
Atomic concentrations were calculated using the relative peak area ratio of the Zr 3d peak and the
metal 2p peak. Assuming a Poisson distribution for the noise in the data, the pooled standard deviation
for each peak area which were converted to atomic concentrations were calculated by implementing
a Monte Carlo simulation through CasaXPS (Casa Software Ltd., Teignmouth, Devon, UK).
4.4. Ink Preparation and Electrochemical Measurements
Each catalyst ink was prepared by dispersing 2.5 mg of carbon black (Vulcan XC-72, Fuel Cell
Store, College Station, TX, USA) and 5 mg of metal-modified ZrP catalyst into 1.53 mL of water, 1.02 mL
of isopropanol, and 10.02 µL of Nafion 117. Carbon black was added to create a conductive surface
since ZrP is not very conductive by itself. The resulting ink was sonicated for 60 min, or longer, until it
was well dispersed. Each catalyst sample was produced by placing a clean, mirror finish-polished,
5 mm diameter glassy carbon disk into a Teflon holder and mounted into an RDE apparatus (Pine
Research Instrumentation, Durham, NC, USA). The RDE was inverted and 10 µL of catalyst ink was
drop-cast onto the disk which was then rotated at 600 RPM for 20–30 min until the catalyst ink dried.
The metal-ZrP loading on the 0.196 cm2 disk was 0.1 mg/cm2 .
The OER activity of the metal-modified ZrP-based catalyst was examined using a three-electrode
RDE setup. The working electrode was composed of a thin coating of catalyst on a glassy carbon disk,
prepared as described above, a Pt wire counter electrode, and an Ag/AgCl reference electrode. The OER
activity was determined by cycling the potential between 1.1 and 1.8 V versus the reversible hydrogen
electrode (RHE) at 20 mVs−1 in O2 -saturated 0.1 M KOH electrolyte and an electrode rotation speed of
1600 RPM. The scan rate was slow enough to ensure steady-state behavior at the electrode surface and
the rotation speed was fast enough to help in product removal from the surface and limit the bubble
formation from oxygen evolution. The solution resistance of the cell was measured at 100 kHz and
ohmic correction occurred after testing. The typical measured solution resistance was ~45 Ω.
Supplementary Materials: The following are available online at www.mdpi.com/2073-4344/7/5/132/s1,
Figure S1: FTIR spectra for adsorbed Fe(III) at a 10:1 M:ZrP ratio; Figure S2: XRPD patterns for Fe(II), Fe(III),
Co(II), and Ni(II)-intercalated ZrP at (from top to bottom) 10:1, 5:1, 3:1, 1:1, 1:3, 1:5, 1:10, and 1:20 M:ZrP molar
ratios. The bottom diffraction pattern in all frames is that of pure α-ZrP; Figure S3: XRPD patterns for Fe(II),
Fe(III), Co(II), and Ni(II)-adsorbed ZrP systems at (from top to bottom) 10:1, 5:1, 3:1, and 1:1 M:ZrP molar ratios.
The bottom diffraction pattern in all frames is that of pure α-ZrP; Figure S4: Cyclic voltammograms of adsorbed
Fe(II), Fe(III), Co(II), and Ni(II)-ZrP systems. The best intercalated 10:1 M:ZrP CV is shown in pink. Dashed lines
represent cyclic voltammograms after 20 cycles.
Acknowledgments: This work was supported by the NSF under NSF Center for Chemical Innovation
CHE-1305124 for Solar Fuels. J.S. was supported by the Department of Defense (DoD) through the National
Defense Science and Engineering Graduate Fellowship (NDSEG) Program. Part of this work was performed
at the Stanford Nano Shared Facilities (SNSF), supported by the National Science Foundation under award
ECCS-1542152. The authors would like to acknowledge Alaina L. Strickler for the TEM micrographs and the
Molecular Sciences Research Center of the University of Puerto Rico.
Author Contributions: T.F.J., J.L.C., M.V.R.-G., and J.S. conceived and designed the experiments; J.S. and M.V.R.-G.
performed the experiments; J.S., I.N., and M.V.R.-G. analyzed the data; and all authors contributed in writing
the paper.
Catalysts 2017, 7, 132
12 of 14
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
Ursua, A.; Gandia, L.M.; Sanchis, P. Hydrogen production from water electrolysis: Current status and future
trends. Proc. IEEE 2012, 100, 410–426. [CrossRef]
Gahleitner, G. Hydrogen from renewable electricity: An international review of power-to-gas pilot plants for
stationary applications. Int. J. Hydrogen Energy 2013, 38, 2039–2061. [CrossRef]
Ehteshami, S.M.M.; Chan, S.H. The role of hydrogen and fuel cells to store renewable energy in the future
energy network—Potentials and challenges. Energy Policy 2014, 73, 103–109. [CrossRef]
Lewis, N.S.; Nocera, D.G. Powering the planet: Chemical challenges in solar energy utilization. Proc. Natl.
Acad. Sci. USA 2006, 103, 15729–15735. [CrossRef] [PubMed]
Gray, H.B. Powering the planet with solar fuel. Nat. Chem. 2009, 1, 7. [CrossRef] [PubMed]
Thomsen, J.M.; Huang, D.L.; Crabtree, R.H.; Brudvig, G.W. Iridium-based complexes for water oxidation.
Dalton Trans. 2015, 44, 12452–12472. [CrossRef] [PubMed]
Browne, M.P.; Nolan, H.; Duesberg, G.S.; Colavita, P.E.; Lyons, M.E. G. Low-overpotential high-activity
mixed manganese and ruthenium oxide electrocatalysts for oxygen evolution reaction in alkaline media.
ACS Catal. 2016, 6, 2408–2415. [CrossRef]
Enman, L.J.; Burke, M.S.; Batchellor, A.S.; Boettcher, S.W. Effects of intentionally incorporated metal cations
on the oxygen evolution electrocatalytic activity of nickel (oxy)hydroxide in alkaline media. ACS Catal. 2016,
6, 2416–2423. [CrossRef]
Burke, M.S.; Enman, L.J.; Batchellor, A.S.; Zou, S.; Boettcher, S.W. Oxygen evolution reaction electrocatalysis
on transition metal oxides and (oxy)hydroxides: Activity trends and design principles. Chem. Mater. 2015,
27, 7549–7558. [CrossRef]
Burke, M.S.; Kast, M.G.; Trotochaud, L.; Smith, A.M.; Boettcher, S.W. Cobalt–iron (oxy)hydroxide oxygen
evolution electrocatalysts: The role of structure and composition on activity, stability, and mechanism. J. Am.
Chem. Soc. 2015, 137, 3638–3648. [CrossRef] [PubMed]
Galán-Mascarós, J.R. Water oxidation at electrodes modified with earth-abundant transition-metal catalysts.
ChemElectroChem 2015, 2, 37–50. [CrossRef]
Antolini, E. Iridium as catalyst and cocatalyst for oxygen evolution/reduction in acidic polymer electrolyte
membrane electrolyzers and fuel cells. ACS Catal. 2014, 4, 1426–1440. [CrossRef]
Fabbri, E.; Habereder, A.; Waltar, K.; Kötz, R.; Schmidt, T.J. Developments and perspectives of oxide-based
catalysts for the oxygen evolution reaction. Catal. Sci. Technol. 2014, 4, 3800–3821. [CrossRef]
Suntivich, J.; May, K.J.; Gasteiger, H.A.; Goodenough, J.B.; Shao-Horn, Y. A Perovskite oxide optimized
for oxygen evolution catalysis from molecular orbital principles. Science 2011, 334, 1383–1385. [CrossRef]
[PubMed]
McCrory, C.C.L.; Jung, S.; Ferrer, I.M.; Chatman, S.M.; Peters, J.C.; Jaramillo, T.F. Benchmarking hydrogen
evolving reaction and oxygen evolving reaction electrocatalysts for solar water splitting devices. J. Am.
Chem. Soc. 2015, 137, 4347–4357. [CrossRef] [PubMed]
Man, I.C.; Su, H.-Y.; Calle-Vallejo, F.; Hansen, H.A.; Martínez, J.I.; Inoglu, N.G.; Kitchin, J.; Jaramillo, T.F.;
Nørskov, J.K.; Rossmeisl, J. Universality in oxygen evolution electrocatalysis on oxide surfaces. ChemCatChem
2011, 3, 1159–1165. [CrossRef]
Weng, B.; Xu, F.; Wang, C.; Meng, W.; Grice, C.R.; Yan, Y. A layered Na1−x Niy Fe1−y O2 double oxide oxygen
evolution reaction electrocatalyst for highly efficient water-splitting. Energy Environ. Sci. 2017, 10, 121–128.
[CrossRef]
Lu, Z.; Wang, H.; Kong, D.; Yan, K.; Hsu, P.-C.; Zheng, G.; Yao, H.; Liang, Z.; Sun, X.; Cui, Y.
Electrochemical tuning of layered lithium transition metal oxides for improvement of oxygen evolution
reaction. Nat. Commun. 2014, 5, 4345. [CrossRef] [PubMed]
Doyle, A.D.; Montoya, J.H.; Vojvodic, A. Improving oxygen electrochemistry through nanoscopic
confinement. ChemCatChem 2015, 7, 738–742. [CrossRef]
Oh, H.-S.; Nong, H.N.; Reier, T.; Bergmann, A.; Gliech, M.; Ferreira de Araújo, J.; Willinger, E.; Schlögl, R.;
Teschner, D.; Strasser, P. Electrochemical catalyst–support effects and their stabilizing role for IrOx
Catalysts 2017, 7, 132
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
13 of 14
nanoparticle catalysts during the oxygen evolution reaction. J. Am. Chem. Soc. 2016, 138, 12552–12563.
[CrossRef] [PubMed]
Wei Seh, Z.; Kibsgaard, J.; Dickens, C.F.; Chorkendorff, I.; Nørskov, J.K.; Jaramillo, T.F. Combining theory
and experiment in electrocatalysis: Insights into materials design. Science 2017, 355. [CrossRef]
Wu, Z.-S.; Yang, S.; Sun, Y.; Parvez, K.; Feng, X.; Müllen, K. 3D Nitrogen-doped graphene aerogel-supported
Fe3 O4 nanoparticles as efficient electrocatalysts for the oxygen reduction reaction. J. Am. Chem. Soc. 2012,
134, 9082–9085. [CrossRef] [PubMed]
Kou, R.; Shao, Y.; Wang, D.; Engelhard, M.H.; Kwak, J.H.; Wang, J.; Viswanathan, V.V.; Wang, C.; Lin, Y.;
Wang, Y.; et al. Enhanced activity and stability of Pt catalysts on functionalized graphene sheets for
electrocatalytic oxygen reduction. Electrochem. Commun. 2009, 11, 954–957. [CrossRef]
Ma, T.Y.; Dai, S.; Jaroniec, M.; Qiao, S.Z. Graphitic carbon nitride nanosheet-carbon nanotube
three-dimensional porous composites as high-performance oxygen evolution electrocatalysts. Angew. Chem.
Int. Ed. 2014, 53, 7281–7285. [CrossRef] [PubMed]
Troup, J.M.; Clearfield, A. On mechanism of ion exchange in zirconium phosphates. 20. Refinement of the
crystal structure of α-zirconium phosphate. Inorg. Chem. 1977, 16, 3311–3314. [CrossRef]
Alberti, G.; Costantino, U.; Gupta, J.P. Crystalline insoluble acid salts of tetravalent metals—XIX:
Na+ -catalyzed H+ -Mg2+ and H+ -Cs2+ ion exchanges on α-zirconium phosphate. J. Inorg. Nucl. Chem.
1974, 36, 2109–2114. [CrossRef]
Alberti, G.; Bertrami, R.; Costantino, U. Crystalline insoluble acid salts of tetravalent metals—XXII: Effect
of small amounts of Na+ on the ion exchange of alkaline earth metal ions on crystalline Zr(HPO4 )2 ·H2 O.
J. Inorg. Nucl. Chem. 1976, 38, 1729–1732. [CrossRef]
Alberti, G.; Costantino, U. Recent progress in the intercalation chemistry of layered α-zirconium phosphate
and its derivatives, and future perspectives for their use in catalysis. J. Mol. Catal. 1984, 27, 235–250.
[CrossRef]
Marti, A.; Colón, J.L. Direct ion exchange of tris(2,20 -bipyridine)ruthenium(II) into an α-zirconium phosphate
framework. Inorg. Chem. 2003, 42, 2830–2832. [CrossRef] [PubMed]
Alberti, G.; Costantino, U.; Gill, J.S. Crystalline insoluble acid salts of tetravalent metals—XXIII: Preparation
and main ion exchange properties of highly hydrated zirconium bis monohydrogen orthophosphates. J. Inorg.
Nucl. Chem. 1976, 38, 1733–1738. [CrossRef]
Atienzar, P.; Victoria-Rodríguez, M.; Juanes, O.; Rodríguez-Ubis, J.C.; Brunet, E.; García, H. Layered
γ-zirconium phosphate as novel semiconductor for dye-sensitized solar cells: Improvement of photovoltaic
efficiency by intercalation of a ruthenium complex-viologen dyad. Energy Environ. Sci. 2011, 4, 4718–4726.
[CrossRef]
Santiago, M.B.; Declet-Flores, C.; Díaz, A.; Vélez, M.M.; Zoé-Bosques, M.; Sanakis, Y.; Colón, J.L. Layered
inorganic materials as redox agents: Ferrocenium-intercalated zirconium phosphate. Langmuir 2007, 23,
7810–7817. [CrossRef] [PubMed]
Santiago, M.B.; Daniel, G.A.; David, A.; Casañas, B.; Hernández, G.; Guadalupe, A.R.; Colón, J.L. Effect of
enzyme and cofactor immobilization on the response of ethanol oxidation in zirconium phosphate modified
biosensors. Electroanalysis 2010, 22, 1097–1105. [CrossRef]
Rivera, E.J.; Barbosa, C.; Torres, R.; Grove, L.; Taylor, S.; Connick, W.B.; Clearfield, A.; Colón, J.L. Vapochromic
and vapoluminescent response of materials based on platinum(ii) complexes intercalated into layered
zirconium phosphate. J. Mater. Chem. 2011, 21, 15899–15902. [CrossRef]
Díaz, A.; Saxena, V.; González, J.; David, A.; Casañas, B.; Carpenter, C.; Batteas, J.D.; Colón, J.L.; Clearfield, A.;
Hussain, M.D. Zirconium phosphate nanoplatelets: A novel platform for drug delivery in cancer therapy.
Chem. Commun. 2012, 48, 1754–1756. [CrossRef] [PubMed]
Horsley, S.E.; Nowell, D.V.; Stewart, D.T. The infrared and Raman spectra of α-zirconium phosphate.
Spectrochim. Acta Part A Mol. Spectrosc. 1974, 30, 535–541. [CrossRef]
Mosby, B.M.; Díaz, A.; Bakhmutov, V.; Clearfield, A. Surface functionalization of zirconium phosphate
nanoplatelets for the design of polymer fillers. ACS Appl. Mater. Interfaces 2014, 6, 585–592. [CrossRef]
[PubMed]
Szirtes, L.; Szeleczky, A.M.; Rajeh, A.O.; Kuzmann, E. Thermal behaviour of transition metal-containing
zirconium phosphate. J. Therm. Anal. Calorim. 1999, 56, 447–451. [CrossRef]
Catalysts 2017, 7, 132
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.
54.
55.
56.
57.
14 of 14
Alberti, G.; Casciola, M.; Costantino, U.; Vivani, R. Layered and pillared metal(IV) phosphates and
phosphonates. Adv. Mater. 1996, 8, 291–303. [CrossRef]
Alagna, L.; Prosperi, T.; Tomlinson, A.A.G.; Ferragina, C.; La Ginestra, A. Extended X-ray absorption fine
structure investigation of local symmetry changes in Co2+ - and Ni2+ -exchanged zirconium phosphates.
J. Chem. Soc. Faraday Trans. 1 Phys. Chem. Condens. Ph. 1983, 79, 1039–1047. [CrossRef]
Alagna, L.; Tomlinson, A.A.G.; Ferragina, C.; La Ginestra, A. Zirconium phosphates partially exchanged
with transition-metal ions: Characterisation and stereochemical changes induced by heat treatment. J. Chem.
Soc. Dalton Trans. 1981, 2376–2384. [CrossRef]
Mani, B.; de Neufville, J.P. Dehydration of chemically and electrochemically impregnated (CI and EI) nickel
hydroxide electrodes. J. Electrochem. Soc. 1988, 135, 800–803. [CrossRef]
Miessler, G.L.; Fischer, P.J.; Tarr, D.A. Inorganic Chemistry, 5th ed.; Pearson Education: London, UK, 2014;
pp. 374–377.
Casañas-Montes, B.; Díaz, A.; Barbosa, C.; Ramos, C.; Collazo, C.; Meléndez, E.; Queffelec, C.; Fayon, F.;
Clearfield, A.; Bujoli, B.; et al. Molybdocene dichloride intercalation into zirconium phosphate nanoparticles.
J. Organomet. Chem. 2015, 791, 34–40. [CrossRef]
Sun, L.; Boo, W.J.; Sue, H.-J.; Clearfield, A. Preparation of α-zirconium phosphate nanoplatelets with wide
variations in aspect ratios. New J. Chem. 2007, 31, 39–43. [CrossRef]
Costantino, U.; Szirtes, L.; Kuzmann, E.; Megyeri, J.; Lázár, K. Exchange of iron ions into layers of α-zirconium
phosphate. Solid State Ion. 2001, 141, 359–364. [CrossRef]
Yamashita, T.; Hayes, P. Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials. Appl. Surf. Sci.
2008, 254, 2441–2449. [CrossRef]
Nemoshkalenko, V.V.; Didyk, V.V.; Krivitskii, V.P.; Senekevich, A.I. Investigation of the atomic charges in
iron, cobalt and nickel phosphides. Zhurnal Neorganicheskoi Khimii 1983, 28, 2182–2192.
Hernández-Huesca, R.; Braos-García, P.; Mérida-Robles, J.; Maireles-Torres, P.; Rodríguez-Castellón, E.;
Jiménez-López, A. Cobalt-based alumina pillared zirconium phosphate catalysts for the selective catalytic
reduction of NO by propane. Chemosphere 2002, 48, 467–474. [CrossRef]
Nardi, K.L.; Yang, N.; Dickens, C.F.; Strickler, A.L.; Bent, S.F. Creating highly active atomic layer deposited
NiO electrocatalysts for the oxygen evolution reaction. Adv. Energy Mater. 2015, 5, 1500412. [CrossRef]
Cheikhi, N.; Ziyad, M. Methane conversion to higher alkanes over ruthenium and cobalt loaded Zr3 (PO4 )4 .
J. Phys. IV 2005, 123, 301–305. [CrossRef]
Zhan, Y.; Lu, M.; Yang, S.; Liu, Z.; Lee, J.Y. The origin of catalytic activity of nickel phosphate for oxygen
evolution in alkaline solution and its further enhancement by iron substitution. ChemElectroChem 2016, 3,
615–621. [CrossRef]
Colón, J.L.; Thakur, D.S.; Yang, C.-Y.; Clearfield, A.; Martin, C.R. X-ray photoelectron spectroscopy and
catalytic activity of α-zirconium phosphate and zirconium phosphate sulfophenylphosphonate. J. Catal.
1990, 124, 148–159. [CrossRef]
Arfelli, M.; Mattogno, G.; Ferragina, C.; Massucci, M.A. XPS characterization of γ-zirconium phosphate and
of some of its intercalation compounds. A comparison with the α-zirconium phosphate analogues. J. Incl.
Phenom. Mol. Recognit. Chem. 1991, 11, 15–27. [CrossRef]
Xie, J.; Wang, R.; Bao, J.; Zhang, X.; Zhang, H.; Li, S.; Xie, Y. Zirconium trisulfide ultrathin nanosheets as
efficient catalysts for water oxidation in both alkaline and neutral solutions. Inorg. Chem. Front. 2014, 1,
751–756. [CrossRef]
Szirtes, L.; Megyeri, J.; Kuzmann, E.; Klencsár, Z. Electrical conductivity of transition metal containing
crystalline zirconium phosphate materials. Solid State Ion. 2001, 145, 257–260. [CrossRef]
Kijima, T. Direct preparation of θ-zirconium phosphate. Bull. Chem. Soc. Jpn. 1982, 55, 3031–3032. [CrossRef]
© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).